The present invention relates to a monitoring and a control system for monitoring the boil states of the contents of a cooking utensil located on a cooking surface of a cooktop and then responding by at least one of providing indication of the state to a user, issuing a signal representative of the state, and controlling the energy applied to the cooking surface.
Recently, standard porcelain enamel cooktop surfaces of domestic ranges have been replaced by smooth surface, high resistivity cooktops located above one or more heat sources, such as electrical heating elements or gas burners. The smooth surface cooktops improve cleanability of the cooktops, because they provide a continuous surface without seams or recesses in which debris can accumulate. The continuous cooktop surface also prevents spillovers from coming into contact with the heating elements, or burners. Such cooktops may be milk-white, opaque, glass ceramic or crystal and glass material sold under various tradenames. Glass ceramic material is used frequently because of its low coefficient of thermal expansion and smooth top surface that presents a pleasing appearance.
Glass ceramic surface cooktops are less thermally efficient than are standard cooktops utilizing metal sheathed electrical resistance heating elements having a spiral configuration. The high thermal mass of the glass ceramic material has a slow thermal response, thereby requiring a longer time to heat up or cool down. The heat is stored in the glass ceramic cooktop as well as in the sheathed heating element and the insulating support block or pad, which typically accompany the heating element. When open coil heaters are used at a spaced distance bellow the cooktop, there is also poor thermal coupling between the heat source and the glass ceramic plate. In order to transfer a requisite amount of heat from an open coil heater to the cooktop, the heat source has to operate at a higher temperature than otherwise, which creates problems, such as poor system efficiency, high heat losses, component overheating and high cooktop temperatures. Glass ceramic cooktops in surface units with open coil heaters also may present a safety hazard in the event the cooktop is broken.
Boiling water or other fluids or foods (generically "liquids") is a common step in cooking. For instance, boiling liquids is one of the most common uses for a range. It is typically desirable to closely monitor the boil phase of the liquid during such processes, i.e., to identify boil phases and boil-dry conditions. In this regard, the pre-simmer phase is generally characterized by a calm liquid and the simmer onset phase is an initial, slow bubbling of the liquid characterized by the appearance of individual bubbles. During the simmer phase, bubbles appear in jets creating the effect commonly referred to as simmering. Finally, in the boil phase, the bubbling of the liquid is generalized, resulting in the familiar turbulence of a boiling liquid. These phases can be identified by experts and experienced cooks.
The boiling state is also characterized by the liquid remaining at a constant maximum "boiling" temperature as increased levels of energy are applied due to the phase transition properties of water. The liquid acts as a heat barrier which leads to changes in the thermal transfer properties of the cooktop and the utensil as the liquid approaches and then reaches the boiling temperature. These thermal properties lead to characteristic features in the thermal or power indicative signals as various boil states are attained.
The boil phase of a liquid is monitored for a number of reasons. First, many cooking processes require that the liquid be attended to upon identification of a particular boil phase, e.g., reducing the heat after the liquid reaches a boil. The boil phase may be monitored to reduce heat after the liquid reaches a boil, either to reduce it to a simmer for cooking purposes or to prevent boil-over. Boil-over can result in a burned-on residue on the cooktop, or, in the case of gas ranges, extermination of the cooking flame.
Another reason for monitoring the boil phase is to prevent a boil-dry condition, which may result in burning of the food, damaging the cooking utensil and potential fire hazards. A still further reason is to provide automation to supplant visual monitoring of the boil phase by the user. Such visual monitoring can interfere with the user's ability to prepare other foods or be otherwise disposed during the heating of the liquid. Moreover, a busy or inexperienced cook may fail to accurately, or in a timely manner, identify a boil phase of interest.
Increasingly, manufacturers seek to provide, and consumers desire to have appliances with a greater degree of automated operation and control. With the increasing affordability of integrating computing power into an appliance, there exists a potential to provide the increased levels of automated control. However, information gathering tools or devices that interact with a computer or microcontroller in monitoring or controlling the operation of the appliance must also have desirable cost and performance attributes.
For cooking appliances generally, and for electric and gas range cooktops specifically, automation or partial automation of control of the cooking process, or monitoring of cooking on a cooktop, has traditionally focused on temperature monitoring or sensing. Various temperature sensors have been proposed for sensing the temperature of a surface heating unit or a cooking utensil positioned thereon or food contents located therein, and for controlling the heat input to the heating unit, based on the sensed temperature. Such sensors have commonly been proposed for use in connection with glass ceramic radiant cooktops, and purport to enable detection and control of cooking states of food within a cooking utensil. The sensors directly monitor temperature of the liquid contents of the utensil, and are frequently coupled to the heating unit control system to provide feedback to the control system.
Food temperature-based sensing systems for range cooktops may indirectly or inferentially provide information regarding a boil state of a liquid contained in a utensil and being heated on the cooktop. However, a method for reliably determining the boil state continues to be a problem in cooktop sensing and control, because the correlation between food temperature and boil state depends on a number of variables including, but not limited to, type of liquid, any additives such as salt which raises the boiling point, and the elevation above sea level which raises the boiling point. Finally, the position of the temperature sensor and its calibration can also have a significant impact on achievable accuracy. The general need then is to develop an approach to boil state determination that is more robust to cooking modalities, vessels used, various user interactions, and other variations, or disturbances, in the equipment or environment.